Bug in gmtmath when using values with units

When I add units to the values I get some weird results
```
gmt math -Q 1  1  DIV = 1
gmt math -Q 1c 1  DIV = 1
gmt math -Q 1c 1c DIV = 2.54
gmt math -Q 1i 1i DIV = 2.54 
gmt math -Q 1  1i DIV = 2.54
gmt math -Q 1  1c DIV = 6.4156 (2.54 x 2.54)
```

**System information**
* Operating system: Win64 
* GMT 6.2